Organic semiconductors are being developed for a number of different applications which can be ascribed to the electronics industry in the broadest sense. The structure of organic electroluminescent devices (OLEDs) in which these organic semiconductors are employed as functional materials is described, for example, in U.S. Pat. Nos. 4,539,507, 5,151,629, EP 0676461 and WO 98/27136.
A development in recent years is the use of organometallic complexes which exhibit phosphorescence instead of fluorescence (M. A. Baldo et al., Appl. Phys. Lett. 1999, 75, 4-6). For quantum-mechanical reasons, an up to four-fold increase in energy and power efficiency is possible using organometallic compounds as phosphorescence emitters. Whether this development will succeed depends on whether corresponding device compositions are found which are also able to implement these advantages (triplet emission=phosphorescence compared with singlet emission=fluorescence) in the OLEDs.
In general, there are still considerable problems in OLEDs which exhibit triplet emission. Thus, the operating lifetime is generally too short, which has hitherto prevented the introduction of phosphorescent OLEDs ito high-quality and long-lived devices. In phosphorescent OLEDs, the matrix material used is frequently 4,4′-bis(N-carbazolyl)biphenyl (CBP). The disadvantages are short lifetimes of the devices produced therewith and high operating voltages, which result in low power efficiencies. In addition, CBP has an inadequately high glass-transition temperature. In spite of all the disadvantages of CBP, it continues to be used as triplet matrix material, since the problems described above are not solved satisfactorily even using alternative matrix materials.
The object of the present invention is therefore to provide carbazole derivatives which do not have the above-mentioned problems and which have, in particular, a higher glass-transition temperature without thus adversely affecting the other device properties. A further object of the present invention is to provide carbazole derivatives which result in improved efficiencies and lifetimes on use as triplet matrix material in OLEDs.
Surprisingly, it has been found that derivatives of CBP and other carbazole derivatives in which the carbazole is substituted in the 2-position by an aromatic or heteroaromatic group exhibit significant improvements here. In particular, this results in derivatives having a significantly increased glass-transition temperature and in longer lifetimes and higher efficiencies in the device without adversely affecting the other electronic properties of the compound. The present invention therefore relates to these materials and to the use thereof in organic electronic devices.
U.S. Pat. No. 6,562,982 discloses derivatives of CBP which are substituted by aryl groups in the 3,6-position as charge-transport compounds for organic electroluminescent devices. The glass-transition temperatures of these compounds are not indicated. However, the aryl substituents in these compounds are conjugated with the nitrogen of the carbazole and thus have a significant influence on the electronic properties of the compound. It is therefore not possible in this way to obtain CBP derivatives having comparable electronic properties to CBP.
JP 2004/288381 discloses carbazole derivatives which are substituted by fluorinated aromatic compounds as triplet matrix materials. The fluorinated aryl substituents here are bonded to the carbazole in the 2- or 3-position. Due to the high electronegativity of the fluorine, however, these substituents have a strong influence on the electronic properties of the molecule.